In the context of the present invention, the term “diagnosis” is used as an overall term for medical determinations which may be based on different problems according to the clinical condition of the patient (of the investigated person) for whom the determination is carried out and which serve for the detection and, in the present case, in particular also for the early detection, the determination of the severity and the assessment of the course, including the therapy-accompanying assessment of the course, and the prognosis of the future course of a disease. What is of particular importance in the present context is that a diagnosis may also be a negative diagnosis in which the presence of a certain disease is made improbable owing to the failure to establish certain features typical of the disease, for example the nondetectability of biomarkers or biomarker combinations associated with the relevant disease in a blood sample of a patient.
Biomarkers which can be found at elevated levels in the case of a plurality of different diseases and therefore by themselves do not permit a positive diagnosis of a specific disease—although as a rule they may also be decisive for the positive diagnosis on inclusion of further clinical or biochemical parameters—are also of great value for the negative diagnosis. Biomarker combinations therefore frequently permit statements which could not be made, or could not be made with the same probability, in the case of an isolated determination of only individual biomarkers or parameters.
The diseases regarding the diagnosis of which the present invention is concerned tend to be slowly developing, chronic neurodegenerative diseases of noninfectious etiology, in particular dementias.
Dementias are designated generally as diseases for which a common feature is the loss of acquired intellectual abilities, especially of memory, and of the normal personality level as a result of brain damage. Dementias are as a rule relatively slowly developing diseases of chronic character. If dementias occur prior to old age, in middle age, they are referred to as presenile dementias and they are differentiated on the basis of the symptoms and cerebropathological changes typical of them, in particular the following diseases or groups of diseases:
Alzheimer's disease (AD) is the most frequent neurodegenerative dementia, accounts for ⅔ of all cases of dementia and is also the practically most important field of use for the present invention. AD is distinguished by three important pathological features which however can be detected with certainty only post mortem: the formation of amyloid plaques and neurofibrillar bundles and the loss of nerve cells (for an overview cf. (1); literature references in the description in the form of numbers relate to the list of references following the description). Amyloid plaques consist of extraneuronal aggregates of the amyloid-β protein, while the neurofibrillar bundles contain mainly tau-protein and neurofilaments. It is presumed that the plaque and neurofibril formation is the cause of the death of nerve cells.
The most important symptoms of AD are increasing impairment of the capacity to register and disturbance of intellectual function in combination with relatively persistent emotional responsiveness, these symptoms being accompanied by further less specific disturbances which make it more difficult to distinguish AD from other forms of dementia.
Observations of AD patients and patients who develop AD in the course of their clinical observation over many years led to the formulation of criteria for mutually distinguishable groups of patients which cover the entire range of    (a) persons without subjective and objective cognitive disturbances (which in the context of the present invention represent the control group) through    (b) patients who complain about subjective diminished cognitive ability but in whom no cognitive deficits can be found (in the context of the present invention, this is the group of “SCD” patients, where “SCD” represents “subjective cognitive disturbances”), further through    (c) patients who have been found to have mild cognitive disturbances and who have been diagnosed with “possible AD” (“pos AD”) where no other dementia-causing diseases are present (in the context of the present invention, this is the group “MCD pos AD”, where “MCD” represents “mild cognitive disturbances”) to    (d) the group of patients with the typical clinical picture for considerable cognitive disturbances which have begun gradually and progress slowly, which patients are diagnosed with “probable AD” when other causes of dementia can be ruled out (in the context of the present invention, this is the group “pr AD”, where the abbreviation represents “probable Alzheimer's”).
Regarding the assignment of persons or patients with subjective and/or objective cognitive disturbances to various groups, reference is additionally made to (2), (3), (4) and (5).
Dementia with lewy bodies (DLB) is the second most frequent cause of a dementia after Alzheimer's disease. Neuropathologically, DLB is characterized by the occurrence of so-called lewy bodies in the brain stem and in the cortex. These lewy bodies predominantly comprise aggregates of the presynaptic protein (α-synuclein) and ubiquitin. The lewy body pathology can be associated to various extents with Alzheimer- and Parkinson-typical neuropathological changes. Thus, in the case of DLB too, the formation of beta-amyloid and senile plaques occurs, but not neurofibril bundles (for an overview, cf. (6)). Lewy bodies are also present in the brain of patients with Parkinson's disease, although in a different distribution.
The key symptoms of DLB are a progressive cognitive disturbance, episodes of confusion with fluctuating attentiveness and awareness, Parkinsonism, frequent falls and syncopes (brief, paroxysmal unconsciousness). The sensitivity and specificity of the diagnostic criteria are high throughout regarding the specificity but in some cases very low regarding the sensitivity. This means that DLB is frequently not diagnosed in day-to-day clinical routine.
Frontotemporal dementia (FTD) is also referred to as Pick's disease and accounts for about 20% of presenile dementias. FTD is in some cases of genetic origin and is among the so-called tauopathies, which are distinguished by overexpression or underexpression of a tau-protein subtype or by the expression of a mutated tau-protein. Neuropathologically, local atrophy of the frontal and/or temporal cortex and of the substantia nigra and of the basal ganglia occurs. This results in speech disturbances of varying severity, a personality change and behavioral abnormalities. Overall, FTD is underdiagnosed with a sensitivity of 93% and a specificity of only 23%, AD representing the most frequent misdiagnosis.
The term vascular dementia (VAD) summarizes diseases in which a dementia is triggered owing to blood flow disturbances in the brain. There are different types of VAD, of which multi-infarction dementia (MID) and subcortical VAD (also referred to as Binswanger's disease) are the most frequent forms.
Binswanger's disease is a slowly progressing dementia which is characterized pathologically by cerebrovascular lesions in the white brain substance. Clinically, this results in behavioral abnormalities, such as agitation, irritability, depression and euphoria, and slightly impaired memory.
Multi-infarction dementia occurs gradually as a result of a plurality of small strokes, also referred to as transient ischemic attacks (TIA), which led to the destruction of brain tissue in the cortex and/or subcortical areas. The strokes may also have remained completely unnoticed, in which case the dementia is the first noticeable consequence. When MID is present, there is a gradual decrease in cognitive abilities, associated with severe depressions, mood variations and epilepsy.
A diagnosis of dementia is carried out nowadays predominantly on the basis of neuropsychological investigations and observation of the development of the disease and its course, using exclusion criteria for certain forms of dementia. In very many cases, these investigations give ambiguous results, which explain the abovementioned numbers for the underdiagnosed forms of dementia or incorrectly diagnosed cases. The cerebral changes typical of the disease cannot of course be determined directly on living patients, and investigations of the brain functions using medical equipment by means of, for example, X-ray tomography or magnetic resonance imaging are complicated and expensive.
It would therefore be desirable to be able to supplement and thereby considerably improve the detection and in particular early detection of dementias by the measurement of informative biomarkers which can be determined, for example, in a blood sample (serum sample, plasma sample) of a patient with the aid of relatively simple test methods.
For the diagnosis of Alzheimer's disease, the Ronald and Nancy Reagan Institute of the Alzheimer's Association and the NIA Working Group published guidelines for the criteria which are set regarding an ideal biomarker for the detection of AD (7). The following criteria should ideally be fulfilled by the biomarker:    1. It should be brain-specific and detect a fundamental feature of the neuropathology of these diseases.    2. The diagnostic sensitivity and the specificity of at least 80% should exist.    3. The disease-specific change of the biomarker should manifest itself in as early a stage as possible of the disease, in order to be able to begin suitable therapeutic measures (8).
Up to the present, however, there is no biomarker which could be used in day-to-day clinical routine in the blood or the cerebrospinal fluid with sufficient certainty for the early and differential diagnosis of AD and fulfills all abovementioned criteria. At present, various potential marker candidates are being investigated, including inflammation markers, such as IL-6 and TNFα, markers for oxidative stress, such as 3-nitrotyrosine, and markers which are associated with characteristic pathological changes of AD, such as amyloid β, which is a main constituent of the amyloid plaques, and the tau-protein, which is a substantial constituent of the neurofibril bundles (cf. the overview in (8); (9)).
There is a current demand for supplementary investigative methods which provide valid laboratory findings and which are based on a determination of substances suitable as biomarkers for dementias, in particular for Alzheimer's disease (AD), in blood or plasma samples and are suitable for supporting an early positive diagnosis and/or for a negative diagnosis by exclusion in the case of persons who are suspected of having a dementia, in particular AD.